1. Field of the Invention
The present invention relates to an electromagnetically-driven shutter or a camera having the electromagnetically-driven shutter, and more particularly to an electromagnetically-driven shutter arranged to use electric energy stored in capacitors, or a camera having such a shutter.
2. Description of Related Art
The shutter of the type to be operated with electric energy stored in capacitors must be arranged to accurately and surely drive electromagnets or the like to lock and unlock a leading blade (a first light blocking structure provided for initiating an exposure) and a trailing blade (a second light blocking structure provided for terminating an exposure) of the shutter. To meet this requirement, it has generally been practiced to have both a leading blade driving capacitor and a trailing blade driving capacitor charged with electricity until immediately before allowing the leading blade to travel for an exposure.
However, since the charging action is performed on the trailing blade driving capacitor only until immediately before the commencement of travel of the leading blade, the level of electric charge stored in the trailing blade driving capacitor comes to drop too much in the event of an exposure over a long period of time, like in the case of celestial photography.
Since it is a property of a capacitor to make a spontaneous discharge, a voltage stored in the capacitor comes to drop when it is left intact for a long period of time. FIG. 4 shows in a graph the characteristic of spontaneous discharge of the conventional capacitor. In FIG. 4, the voltage is shown on the ordinate axis and time is shown on the abscissa axis of the graph. A curve 31 represents a spontaneous discharge taking place at temperature of 25.degree. C. while a curve 32 represents a spontaneous discharge taking place at temperature of 85.degree. C. As apparent from this graph, the voltage drops with the lapse of time. The voltage drop becomes faster accordingly as temperature is increased by a high ambient temperature or internal heat build-up of component elements in use. Therefore, the capacitor must be recharged to replenish it with energy to make up for the amount of a loss caused by the spontaneous discharge.
Further, it is conceivable to prevent such a loss of energy by charging the trailing blade driving capacitor further until immediately before the trailing blade is allowed to travel after the travel of the leading blade while the leading blade driving capacitor is charged until immediately before the travel of the leading blade.
According to this charging method, however, the trailing blade driving capacitor is continuously charged until immediately before the travel of the trailing blade even when its spontaneous discharge has not yet begun. Besides, in the event of a long exposure, such as bulb photography or the like, a charging process tends to be carried on over an unnecessarily long period of time to cause a mere waste of electric energy.